1. Field of the Invention
The present invention relates to power converters. More specifically, the present invention relates to start-up circuits that provide a start-up voltage to a control system of a power converter during a power-up process when an output voltage rises from zero to a nominal level.
2. Description of the Related Art
Conventional IC controllers used in switching power converters have embedded, internal start-up circuits that provide power to the control system of the power converter during startup. An example IC controller is the LM50XX series from Texas Instruments, Inc. (TI). A simplified block diagram of a start-up circuit 100 for a converter 105, which includes an IC controller 101, is shown in FIG. 1. The start-up circuit 100 includes the IC controller 101, and the IC controller 101 includes an internal start-up circuit (not explicitly shown in FIG. 1) that includes the input pin Vin that is typically connected to the terminal Vin+. The IC controller 101 includes an under-voltage-lockout (UVLO) feature. If the input voltage Vinput is below the UVLO voltage, the IC controller is turned off, and if the input voltage Vinput is above the UVLO voltage, the IC controller is turned on. The UVLO voltage is right below the minimum operating voltage of the IC controller 101 at which the IC controller provides an output. When the input voltage Vinput across the terminals Vin+, Vin− of the start-up circuit 100 in FIG. 1 is greater than the minimum operating voltage of the IC controller 101 at the input pin Vin, the output voltage Vcc of the IC controller 101 is enabled to charge an external capacitor 102 connected to the power-supply pin Vcc. The external capacitor 102 is an energy-storage capacitor that provides the initial start-up energy for the power converter 105. A controller output 103 that controls the power transistors of the converter 105 is enabled when the voltage at the power-supply pin Vcc reaches the regulation point (i.e., when the voltage reaches a voltage at which the converter 105 can provide regulation), which is typically 10 V. In a typical application, an auxiliary output 107 of the converter 105 is connected to the power-supply pin Vcc through a diode 104, which is reverse biased with the power-supply pin Vcc during startup.
When the output voltage of the converter 105 of the load terminal 106 reaches its nominal level, the auxiliary output 107 also reaches its nominal level, typically 12V, causing the diode 104 to become forward biased, which shuts off the internal start-up circuit of the IC controller 101. Accordingly, the internal start-up circuit of the IC controller 101 only operates during relatively short times such as during startup and hiccup conditions. Hiccup conditions can be caused by, for example, over-load, over-voltage, and over-temperature conditions. Therefore, steady-state power dissipation in the internal start-up circuit of the IC controller is eliminated. Powering the IC controller 101 from the auxiliary output 107 of the converter 105 through the diode 104 eliminates the internal start-up circuit losses during steady state conditions, reducing overall controller power consumption and improving overall efficiency.
Power consumption P of the start-up circuit 100 during the hiccup condition ON time Ton and with period T, which is the total time between hiccup conditions, can be calculated according to the formula:P=Vin*Ireg*Ton/T,   (1)where Vin is the input voltage across the terminals Vin+, Vin− and Ireg is the start-up regulator current. Even though power consumption P according to formula (1) increases with the input voltage Vinput, it is relatively low because of the low hiccup-condition duty cycle D=Ton/T, which is typically 5%. For example, in the worst-case, power consumption P for the start-up circuit 100 with Ireg=20 mA=0.02 A, at high input voltage Vinput=75 V under hiccup condition with hiccup-condition duty cycle D=Ton/T=0.05 (i.e., 5% hiccup-condition duty cycle) according to formula (1) is: P=75 V*0.02 A*0.05=0.075 W=75 mW.
The above-described internal start-up circuit of the IC controller 101 is efficient in a specified input voltage range, for example, 14 V to 100 V. However, when the maximum input voltage of the converter 105 is greater than the maximum allowable input voltage of the IC controller 101 (typically 100 V at the input pin Vin), the internal start-up circuit of the IC controller 101 cannot be connected to the terminal Vin+. Accordingly, it is recommended (see, for example, the datasheet for the LM5027 controller from TI) to disable the internal start-up circuit of the IC controller 101 by disconnecting the input pin Vin from the terminal Vin+ and providing, as illustrated in FIG. 2, an external high-voltage start-up circuit 208 to provide the power-supply voltage Vcc.
The other parts and basic operation of the circuit 200 in FIG. 2 are the same or substantially the same as in the circuit 100 in FIG. 1. The IC controller 201 is enabled to charge an external capacitor 202 connected to the power-supply pin Vcc. The converter 205 includes auxiliary output 207 and load terminal 206. The converter 205 is controlled by the controller output 203. The auxiliary output 207 of the converter 205 is connected diode 204.
Because the internal start-up circuit of the IC controller 201 is disabled in high-input-voltage applications according to FIG. 2, an IC controller 201 without an internal start-up circuit can also be used provided that the power-supply voltage Vcc is supplied by an external high-voltage start-up circuit 208.
An example of an efficient start-up circuit with no losses at steady state that can be used as an external start-up circuit 208 in FIG. 2 is disclosed in U.S. Pat. No. 8,427,849. The external start-up circuit of U.S. Pat. No. 8,427,849 is shown in FIG. 3 as start-up circuit 300 The external start-up circuit 300 is connected to the power converter 307 and IC controller 308. The power converter 307 includes an auxiliary output terminal 309. The external start-up circuit 300 includes a start-up transistor 301, a first diode 302, a resistor 303, a Zener diode 304, a capacitor 305, a second diode 306, and a switch Q with a control circuit 310 that receives a control signal from the auxiliary output terminal 309 of the power converter 307.
The external start-up circuit 300 in FIG. 3 operates in the following manner. After the input voltage Vinput is applied across the terminals Vin+, Vin−, the Zener diode 304 is activated through the resistor 303 and the switch Q that is initially ON. The transistor 301 is a bipolar junction transistor (BJT) that supplies a start-up voltage to the power-supply pin Vcc of the IC controller 308 and across the capacitor 305 equal to the zener voltage Vz of the Zener diode 304 minus the combined voltage drop of the base-to-emitter voltage drop of the transistor 301 and the voltage drop across the first diode 302. The start-up voltage, applied to the power-supply pin Vcc of the IC controller 308, reverse biases the second diode 306 and enables the controller output 311, which initiates startup of the power converter 307. During start-up, the power converter 307 increases the output voltage on the LOAD and the auxiliary voltage on the auxiliary output 309 to their nominal levels. The start-up current for the control circuit 308 is supplied by the transistor 301. The transistor 301 is controlled independent of the input voltage Vinput by the fixed zener voltage Vz of the Zener diode 304, resulting in a fixed start-up time over the entire input voltage range.
When the auxiliary voltage at the auxiliary output terminal 309 reaches a predetermined voltage, the control circuit 310 switches switch Q OFF, causing the transistor 301 to also switch OFF. Once the transistor 301 switches OFF, the first diode 302 becomes reverse biased, the second diode 306 becomes forward biased, and the auxiliary power from the auxiliary output 309 supplies power to the power-supply pin Vcc of the controller 308. Accordingly, the supply voltage supplied to the power-supply pin Vcc of the controller 308 at start-up is determined by the zener voltage of the zener voltage Vz (typically 10 V) and at steady state is provided by the auxiliary voltage from the auxiliary output 309 (typically 12 V). The maximum-allowable high input voltage across the terminals Vin+, Vin− is limited by the transistor 301 and by the switch Q in contrast to the start-up circuit 100 in FIG. 1 that is limited by the internal start-up circuit of the controller 101. Power consumption in the start-up circuit 300 in FIG. 3 at steady state is eliminated because the resistor 303 is not conducting and transistor 301 is in the OFF state.
Power consumption P of the start-up circuit 300 in FIG. 3 under hiccup conditions can be calculated asP=Vin*Ireg*Ton/T+Vin*[(Vin−Vz)/R]*Ton/T,   (2)where T and Ton, as in formula (1), are the period and the ON time of the hiccup condition, R is the resistance value of the resistor 303, and Vz is the zener voltage of the Zener diode 304. Equation (2) has the form P=A+B, where A is the same as formula (1) and B is a function of the input voltage Vinput. Assuming the same basic parameters as in the above example: Vin=75 V, Ton/T=0.05, and Ireg=20 mA, Vz=12 V, R=3 kΩ, equation (2) provides:P=75 V*0.02 A*0.05+75 V*[(75 V−12 V)/3000 Ω]*0.05=0.15 W=150 mW,which is 2 times greater than the power consumption calculated above for the start-up circuit 100 in FIG. 1. Drawbacks of the external start-up circuit shown in FIGS. 2 and 3 are increased power consumption at high input voltages under hiccup conditions and increased physical size, complexity, and cost.
Thus, there is a need for a more efficient, less complex, and less expensive start-up circuit for high-input-voltage power converters that preferably uses the internal start up circuit of an IC controller.